In a continuous casting process of steel, a cast slab is manufactured by: cooling a molten steel in a casting mold to produce a solidifying shell; then pulling out the produced solidifying shell to cool at a secondary cooling zone; and solidifying the shell until solidified in center. At this time, in order to control a surface temperature of the cast slab to have an appropriate value so as to inhibit generation of surface cracking and segregation of cast slab, heretofore, a measurement of the surface temperature of the cast slab at the secondary cooling zone of a continuous caster has been carried out.
As points at which the surface temperature of the cast slab is to be measured at the secondary cooling zone of the continuous caster, there are many points in a longitudinal direction (a casting direction) and a width direction of the cast slab having distances from each other. Also, in order to measure the surface temperature of the cast slab continuously, it is desirable to employ a contactless thermometer such as a radiation thermometer, however, cooling water sprayed at the secondary cooling zone becomes to be a disturbance factor. Therefore, problems as below exist.
<Error in temperature measurement due to absorption of light by water>
A thermal radiation light from a surface of the cast slab to be detected is absorbed by water existing in a light path in between the cast slab and the radiation thermometer. Because of this absorption, an error in temperature measurement can occur, and sometimes the temperature cannot be measured.
<Error in temperature measurement due to light scattering occurred by water drop>
The thermal radiation light from the surface of the cast slab to be detected is scattered and decayed by water drops existing in the light path of the radiation thermometer (dropped water from support rolls that support the cast slab and cooling water have contact with the cast slab to evaporate, after that the water vapor is condensed to be water drops in the form of a mist). Because of this scattering, an error in temperature measurement can occur, and sometimes the temperature cannot be measured. Also, in a continuous caster of a vertical bending type, a first half of the continuous casting process has a vertical pass line, that is, a measurement face of the cast slab is in a vertical position, and a last half of the continuous casting process has a horizontal pass line, that is, a measurement face of the cast slab is in a horizontal position. Therefore, influence from the cooling water to temperature measurement differs depending on places of the temperature measurement.
<Concern of ingress of large amount of water into nozzle when casting is started>
Conventionally, in order to inhibit influence from water and water drops existing in the light path of the radiation thermometer, a method of jetting purge air from nozzles toward a target of temperature measurement has been suggested. On the other hand, in a continuous caster, a large amount of water flows in a time period from before the casting is started till the very first stage after the casting is started and the like. Specifically, nowadays, in the vertical pass line in the first half of the continuous casting, a large amount of sprayed cooling water falls down. Because of this, there is a possibility of ingress of water into the nozzles that jet the purge air. Amore specific description will be made as below. As a control of cooling water, in every section that is generally called a segment, spraying of cooling water is switched on and off and the amount of the cooling water is adjusted, based on a setting to each segment. Therefore, in a very first stage after the casting is started, when a part of a cast slab reaches to a segment, a certain amount of cooling water is sprayed in the segment overall. Then, in a downstream part of the segment, in which the cast slab has not entered, the cooling water does not hit the cast slab but run down. For example, when the cast slab has reached to an area that covers ⅓ from a most upstream side of cooling zone called as a top zone of directly below of a casting mold, a remained area of ⅔ on a downstream side from the top zone does not contribute to cooling of the cast slab. A large amount of water sprayed to the area that does not contribute to cooling of the cast slab possibly enters into the nozzles for purge air of the radiation thermometer installed lower than the area. Especially, nowadays, methods to strongly cool a cast slab at a most upper portion of a secondary cooling zone of a continuous caster and the like have been developed, thus an influence from a large amount of sprayed water falling down has becoming larger than ever before.
<Problem accompanied by temperature measurement at many points>In order to inhibit surface cracking of cast slab made of Ni steel and the like in a continuous caster of a vertical bending type, conditions of strain rate and surface temperature of a cast slab at a bending part and correcting part of the continuous caster need to be adequately controlled. The strain rate is decided by structure (curvature) of each part and casting speed. The structure of each part is fixed, and it is difficult to change the casting speed since the casting speed controls productivity. Therefore, surface temperature of the cast slab at each part needs to be adequately controlled. However, there are a lot of changing factors that cannot be detected, such as degradation of cooling sprays at the secondary cooling zone, variation of heat transmission parameters such as water temperature and amount of cooling water, component and temperature distribution in a width direction of the cast slab, temperature variation and flux in a casting mold of molten steel. Therefore, it is difficult to obtain an accurate surface temperature of the cast slab at required portion by a cooling model and the like. Because of this, surface temperature measurement of a cast slab at or near each portion with a high accuracy is a very effective means.
Also, by measuring the surface temperature at only one point of the cast slab to reflect the temperature measurement value to the cooling model, it is difficult to obtain the surface temperatures of other portions with a sufficient accuracy. The reason is that since the surface temperature of the one point is influenced by a lot of parameters as described, even when a same surface temperature is detected, the temperature distribution in a thickness direction of the cast slab and a thickness of a solidifying shell are sometimes different, and in such a case, surface temperature of the cast slab at a point located lower side than the point where the surface temperature is measured is to be different from each other. As described below, it is insufficient to measure temperature at only one convenient point of the cast slab, but it is needed to measure temperatures of a plurality of points in a casting direction. It is also considered that if steel grade of the cast slab or pulling out speed of steel is changed, the optimal position in the casting direction where the surface temperature is to be measured in order to inhibit surface cracking is changed. From this viewpoint as well, it is desirable to carry out temperature measurement at a plurality of points in the casting direction.
Further, since temperature distribution exists in the width direction of the cast slab, it is also needed to measure surface temperatures of a plurality of points in the width direction of each part. Regarding the width direction of the cast slab, temperature measurements of at least points located near both ends and central portion are needed. For example, when a shaft bearing of a support roll that supports the cast slab exists in an area corresponding to ⅓ of width of the cast slab from the edge of the cast slab in the width direction, sometimes only the 2 points in the width direction near the shaft bearing have different temperatures from other points. Further, an area corresponding to ⅓ to ¼ of the width from the edge of the cast slab in the width direction may have a higher temperature than a surrounding area. Considering this, the temperature measurement is needed at 5 to 7 points in the width direction. Therefore, it is desirable to carry out the temperature measurement at 2 points in the casting direction, and for example 6 points in the width direction, on both sides of the cast slab (temperature measurement at 24 points in total). As described above, it is needed to carry out the temperature measurement at many points having distances.
In order to measure the surface temperatures at many points having distances, many radiation thermometers are needed, thereby a large amount of initial cost is needed including installation work of the radiation thermometers. For instance, in order to install 12 of radiation thermometers, a large amount of initial cost of (cost of the radiation thermometers+cost of installation work+cost of additional devices)×12 is needed. Also, considering a possibility of breakdown of the radiation thermometers, there is a need to prepare extra radiation thermometers, thereby more radiation thermometers are needed than that to be actually installed to the continuous caster. Also, when many radiation thermometers are used, its maintenance becomes troublesome, and that results in increasing running cost. Confirmations of accuracy and normal operation (checking) are required to the radiation thermometers periodically such as from every half year to every several years. In order to check the radiation thermometer, some work such as removing the radiation thermometer from the installment point and confirming the accuracy using a blackbody furnace and a reference radiation thermometer is needed. It is desirable to set a plurality of temperature points to the blackbody furnace representing temperatures to be measured, however, a certain amount of time is needed to carry out this checking work to many radiation thermometers, thereby cost for the checking also increases. Also, it is difficult to complete the checking work to all radiation thermometers in a short maintenance term of the continuous caster. Considering this, it is presumed that nearly same numbers of extra radiation thermometers as the radiation thermometers that are actually installed are needed, therefore cost becomes further increased.
<problem in a case where nozzles are made to be closer to surface of cast slab>
There is a problem caused by a structure of vertical bending type of a continuous caster of recent years. In a continuous caster of a vertical bending type, a dummy bar (a plurality of blocks made by steel connected to one another) is disposed inside before casting is started to be used as a bottom cap when the casting is started. At the same time of starting of casting, the dummy bar has a role to lead pulling out of a cast slab downward. When the dummy bar leads pulling out, a strong tension is applied to the dummy bar by a pinch roll. In a bending part of the continuous caster, the dummy bar passes the part bending at an axis of a connecting portion of the blocks. Because of this, sometimes the bent portion of the dummy bar widely deviates outside from the external tangent line of adjacent support rolls (external tangent line on a side of cast slab). Therefore, if the nozzles for purge air are projected to near the surface of the cast slab, the dummy bar possibly hits the nozzles. If the dummy bar hits the nozzles, by being caught up in the support rolls, the nozzles bend and thereby the radiation thermometer gets broken, or the bent nozzles damage the support rolls and the damage is sometimes transferred to the cast slab. This makes a great negative effect to a quality and productivity of the cast slab.
As a provision of preventing collision of the dummy bar and the nozzles, it is considered to provide a moving structure to evacuate the nozzles. However, since inside of the continuous caster is hot and humid, such a moving structure easily gets broken, thereby it is difficult to stably use the structure for a long period of time. Also, installation of a large moving structure in Which an sufficient provision is given for preventing breakdown is sometimes difficult considering space, or possibly disturbs maintenance capacity and the like of the continuous caster.
Considering different problems as described above, for example, Patent Document 1 suggests a surface temperature measuring apparatus of slab comprising a first pipe in which an optical fiver is inserted and a space formed by covering a backward portion of the first pipe by a second pipe to circulate cooling water (claims of Patent Document 1). Then, Patent Document 1 describes that the backward portion of the first pipe is connected to a compressed air source and air sent from the backward are jetted from a top end of the first pipe. (Patent Document 1, the second page, left column, line 16 to right column, line 8). It is also described that a copper pipe having good heat conductivity is inserted to an inner surface of top end of the first pipe (Patent Document 1, second page, left column, line 9 to line 15). Further, it is described that by inserting a thin portion that is configured only by the first pipe to a gap between support rolls of the continuous caster to circulate cooling water in the space, it is possible to cool to the top end of the first pipe, therefore it is possible to protect the optical fiber from heat, and by jetting air from the top end of the first pipe, it is possible to prevent ingress of water drops, and at the same time, since water vapor existing in sight is blown away to open the sight, it is also possible to certainly enter the thermal radiation light radiated from the surface of the slab to the top end of the optical fiber (Patent Document 1, second page, left column, line 19 to right column, line 8).
However, the apparatus described in Patent Document 1 is an apparatus to measure a surface temperature at only one point of a cast slab, therefore it is not an apparatus that resolves the problems accompanied with the temperature measurement at many points described above. Also, when the invention described in Patent Document 1 was made, main stream type of the continuous caster was of a vertical type or a bending type, and pass lines of cast slab make a straight line or a curve having a constant curvature. Therefore, it is presumed that there has not been considered of clash of nozzles for purge air and a dummy bar at all.
Also, Patent Document 2 suggests a cooling control method of a continuous cast comprising the steps of, in a secondary cooling zone of a continuous cast equipment, detecting a temperature distribution in a width direction on a surface by a temperature measurement structure in the width direction of a cast slab, carrying out adjustment of flow amount of cooing water automatically by a flow amount adjustment structure via an arithmetic device that controls the temperature distribution in the width direction on the surface to correspond to an intended temperature, thereby controlling temperature of the cast slab (Patent Document 2, claim 1). In Patent Document 2, there is no description of configuration of the thermometer itself. Intended continuous caster of the method described in Patent Document 2 is of a bending type, and a correction point of the cast slab is only one point at a horizontal pass line close to the end of process in a casting direction (Patent Document 2, FIG. 1 and the like). A thermometer is disposed to the correction point, and moving the thermometer in the width direction of the cast slab by using the temperature measurement structure, the temperature distribution in width direction on the surface is measured (Patent Document 2, second page, left below column, line 3 to right below column, line 8).
However, the method described in Patent Document 2, in the same manner as in the case where a moving structure to evacuate the nozzles as described is disposed, has problems of breakdown and having a large space, since a structure to move the thermometer in the width direction is installed. Specifically, in a case where a surface temperature of cast slab is measured in a continuous caster of a vertical bending type, if such a structure is installed to a vertical pass line, it is considered that stable operation of the structure is further disturbed since the structure is subjected to cooling water including foreign substance such as a large amount of oxide scale peeled off from a surface of the cast slab.
Patent Document 3 suggests a cooling apparatus installed inside a roller apron to cool a camera type thermometer for slab surface that measures a temperature of slab surface, wherein a water cooling jacket is provided to a circuit of the camera type thermometer for slab surface, a skirt having a folding-fan shape is provided to a lower portion of the water cooling jacket, an air spray nozzle having a ring shape and many jet holes on side surface and lower surface is provided on the upper side of the camera type thermometer for slab surface in the water cooling jacket, and an air spraying device formed by a member having a shape of straight line attached to one side of the skirt and located inside the skirt, the air spraying device forming an air membrane laterally or obliquely downward (Patent Document 3, claim 1). According to this cooling apparatus, since the camera type thermometer for slab surface is equally cooled efficiently, which eliminates a negative pressure, it is possible to inhibit taking in of water vapor from the slab surface, and since the water vapor coming from the slab is removed and sight of the camera type thermometer for slab surface is secured, radiation heat from the slab also can be inhibited (Patent Document 3, second page, left column, line 24 to line 34).
However, although the skirt described in Patent Document 3 can be applied at a pass line which is nearly in a horizontal position where the camera type thermometer sees downward, in a case where the pass line is vertical in a continuous caster of a vertical bending type, that is, a surface of cast slab is nearly in a vertical position, and a direction of air purge is nearly horizontal, a large amount of water flows in a time period from before the casting is started till the very first stage after the casting is started and the like, and neighborhood of the skirt is in a state being nearly sunk in water. At that time, if such a large skirt having a folding-fan shape as described in Patent Document 3 is used, uniformity in flow amount of air is disrupted and there is a high possibility of taking in water into the skirt from some part. Also, sometimes there is no space to install such a large skirt.
Also, Patent Document 4 suggests a surface temperature measuring method of cast slab in a continuous caster by means of a radiation thermometer, the method comprising measuring a surface temperature of cast slab by the radiation thermometer in a state that spray of a second cooling water to an intended area of temperature measurement on the surface of cast slab is temporarily stopped to thereby inhibit generation of water vapor from the second cooling water in the intended area of temperature measurement on the surface of the cast slab (Patent Document 4, claim 1).
However, in the method described in Patent Document 4, since spray of cooling water is temporarily stopped when temperature measurement is carried out, amount of heat extraction from the surface of the cast slab is largely different between when the temperature measurement is carried out and when the temperature measurement is not carried out, and there is a possibility that the surface temperature of the cast slab is high when the temperature measurement is carried out. The temperature difference between when the temperature measurement is carried out and when the temperature measurement is not carried out differs depending on different conditions. That is, representativeness of values of temperature measurement is vitiated. Also, nowadays, a method to strongly cool a cast slab at a most upper portion of a continuous caster and the like have been developed, and in an area where some of a large amount of cooling water used at the most upper portion (pass line nearly in a vertical position), water that influences value of temperature measurement cannot be sufficiently eliminated only by temporarily stopping the spray of cooling water between specific support rolls, and sometimes conditions cannot be satisfied to enable the measurement.